Filament lead-in and impedance matching structure for a grounded grid amplifier



Dec. 25, 1956 s rr ETAL 2,775,660

FILAMENT LEAD-IN AND IMPEDANCE MATCHING STRUCTURE FOR A GROUNDED GRID AMPLIFIER Filed Dec. 2, 1953 4 Sheets-Sheet 1 Y/ZAEW A? 5 02 2 B a. a ATTORNEY Dec. 25, 1956 H. R. SMITH ET AL FILAMENT LEAD-IN AND IMPEDANCE MATCHING STRUCTURE Filed Dec. 2, 1953' FOR A GROUNDED GRID AMPLIFIER 4 Sheets-Sheet 2 Dec. 25, 1956 H. R. SMITH ET AL 2,775,660

FILAMENT LEAD-IN AND IMPEDANCE MATCHING STRUCTURE FOR A GROUNDED GRID AMPLIFIER Filed Dec. 2, 1955 4 Sheets-Sheet s [PIZTE L635: IMPEDANCE L J T LEM 29 :2

3'7- INVENTORS AMii/RfiM/fl/ A T TORNE Y p Dec. 25, 1956 H. R. SMITH ET AL 4,775,660

FILAMENT LEAD-IN AND IMPEDANCE MATCHING STRUCTURE FOR A GROUNDED GRID AMPLIFIER Flled Dec. 2, 1955 4 Sheets-Sheet 4 .TRANSFORMER gON DU GTOR TO CONDUCTOR TO FIL. TRANSFORMER fllnited States Patent LEAD-1N AND IMPEDANCE MATCH- .ING STRUCTURE FOR A GROUNDED GRID AMPLIFIER Harry R. Smith, Verona, and Allen R. Taylor, Belleville, N. J., assignors to Standard Electronics Corporation, Newark, N. J., a corporation of Delaware Application December 2, 1953, Serial No. 395,796

3 Claims. (Cl. 179-171) This invention relates to a structure for applying filament heater power to a vacuum tube and at the same time matching the input impedance of the vacuum tube to an input coaxial transmission line. The vacuum tube in this case is operated as a cathode driven or grounded grid amplifier.

A primary object of the invention is the provision of a structure for accomplishing the above functions which also affords a convenient access to the vacuum tube whereby the same may readily be disconnected at the filament lead-in. for removal and replacement in the grounded grid amplifier structure.

Briefly described the structure comprises an input connecting means for a coaxial transmission line which is thereby coupled to the cathode of a vacuum tube for a grounded grid amplifier. A pair of coaxial, impedance matching stubs for the coaxial line are provided so that the latter may eflectively be terminated in its characteristic impedance by proper adjustment of the matching stubs. The heater power for the filament is introduced from a source, such as a filament transformer, to the vacuum tube by conductors which pass through the hollow, inner conductors of the matching stubs.

In order to provide for convenient disconnection of the filament connectors, the latter are hinged about the same axis as that of the input coaxial transmission line. The hinge member is constructed so as to furnish a cathode and filament capacitive by-pass.

There are additional advantages derived from the structure of this invention for providing an input impedance to a grounded grid amplifier which may be made to terminate a coaxial feed line in its characteristic impedance. Thus, input impedance remains essentially constant and resistive over a considerable band of frequencies so that the arrangement is ideally suited to broad band television amplifier applications.

Since the input impedance to the amplifier can, by this invention, be made equal to the output load impedance, such an amplifier can be patched out; that is, the input leads can be switched to the output load, in the event of amplifier failure so that lost air time can be minimized in commercial broadcasting.

In view of the fact that the input impedance can be made equal to the load impedance of a transmitter, such as an antenna, higher power amplifiers can be inserted between existing transmitters, which may be of low power, and the existing antenna input terminals with a minimum amount of modification required of such existing equipment.

A vestigial side band filter, such as is often used in television broadcasting, which requires a resistive termination for proper operation can be placed in the input side of an amplifier, by virtue of this invention, rather than the output as is ordinarily done, and smaller filter elements may be used at the resulting lower power level.

Reference is made to the drawings in which Fig. l is a vertical cross section of a radio frequency amplifier Patented Dec. as, 1956 construction suitable to embody the filament lead-inv structure of this invention.

The broad band tuned circuit structure comprises primary and secondary tuned coaxial line sections, the outer conductors 43 and 44 respectively of which are, joined together, as by welding, at 45. There is a communicating slot 46 between the primary and secondary tuned coaxial line sections, the outer conductors 43 and 44 respectively of which are joined together, as by welding, at 45. There is a communicating slot 46 between the primary and secondary tuned coaxial line sections. The vacuum tube is mounted in the upper end of the inner conductor 47 of the primary section. The inner conductor of the secondary section is shown. at 48. The tuning means for the circuit comprises shorting rings 49 and 50 respectively for the primary and secondary sections. The shorting rings are connected together by a bar 51 which. extends through the slot 46. There is also a mutual coupling devices 52 extending through the slot 46 which makes sliding contact with the inner conductors 47 and 48. The mutual coupling device 52 and the shorting rings 49 and 50 a sprocket 54' fixed thereto.

Anode potential for the vacuum tube is introduced by means of th conductor 55 located within a tubular element;

56 (forming a radio frequency block) inside the inner conductor 47 of the primarytuned section.

Other elements of the broad band tuned circuit struc ture are a slack take-up sprocket 57 for the chain drive and also an adjustable tracking impedance comprisinga tubular inductance element 58 and fixed and movable plates 59 and 60 respectively of a variable condenser, all of which are more fully described in co-pending application Serial No. 312,234, filed September 30, 1952. An adjustable coupling capacitor 61 couples the radio frequency energy Within the secondary tuned coaxial transmission line section 44, 48 to an output transmission line comprising inner and outer conductors 62 and 63 respectively. The output from the broad band tuned circuitis led downwardly and outwardly through the inner conductor 48 of the secondary tuned coaxial section by means of the transmission line 62, 63.

Fig. 2 is an enlarged fragmentary view of a portion of Fig. 1 showing the filament lead-in structure of this invention;

Fig. 3 is a top plan view of the structure of Fig. 1;

Fig. 4 is a plan view corresponding to Fig. 3 with the cover removed showing in greater detail, and partly in cross-section, the hinged connector portion of the filament lead-in;

Fig. 5 is a simplified showing of the structure of the essential elements of this invention;

Fig. 6 is a schematic diagram which illustrates a principle of this invention and is substantially the open wire equivalent of Fig. 5.

Fig. 1 of the drawings shows an amplifier construction embodying a broad band tuned circuit which forms the subject matter of copending application Serial No. 312,234, filed September 30, 1952. Figs. 3 and 4 show at 10 and 11 sections of transmission line which serve to interconnect the incoming line 24 and matching stubs 25 and 27 as shown in Fig. 5. The sections 10 and 11 and the T and elbow which make up the stubs 25 and 27 of Fig. 5 are preferably standard transmission line parts such as a 1 /8 transmission line would call for.

With reference to Fig. 1, only the upper portion of the figure underneath the cover 40 is of particular pertinence to the input arrangement of this invention. In. order to show a typical environment for the invention, however, the principle elements of a broad band, tuned circuit amplifier having a grounded grid, in which the invention has been incorporated, will be briefly described.

The transmission line sections and 11 (only the line section 10 can be seen in Fig. 1) convey the radio frequency input and filament power through the capacitive element 12 to vacuum tube connectors 18 and 20. The vacuum tube is indicated in Fig. 1 in dotted lines below the connectors. The cover 40 is hinged at 41 and a latch is shown at 42.

The broad band tuned circuit structure comprises primary and secondary tuned coaxial line sections, the outer conductors 43 and 44 respectively of which are joined together, as by welding, at 45. There is a communicating slot 46 between the primary and secondary tuned coaxial line sections. The vacuum tube is mounted in the upper end of the inner conductor 47 of the primary section. The inner conductor of the secondary section is shown at 48. The tuning means for the circuit comprises shorting rings 49 and 50 respectively for the primary and secondary sections. The shorting rings are connected together by a bar 51 which extends through the slot 46. There is also a mutual coupling device 52 extending through the slot 46 which makes sliding contact with the inner conductors 47 and 48. The mutual coupling device 52 and the shorting rings 49 and 50 can be adjustably spaced by means of a screw 53 and the shorting ring and mutual coupling device are moved in unison by rotation of a chain drive shaft 54, having a sprocket 54 fixed thereto.

Anode potential for the vacuum tube is introduced by means of the conductor 55 located within a tubular element 56 (forming a radio frequency block) inside the inner conductor 47 of the primary tuned section.

Other elements of the broad band tuned circuit structure are a slack take-up sprocket 57 for the chain drive and also an adjustable tracking impedance comprising a tubular inductance element 58 and fixed and movable plates 59 and 60 respectively of a variable condenser, all of which are more fully described in co-pending application Serial No. 312,234, filed September 30, 1952. An adjustable coupling capacitor 61 couples the radio frequency energy within the secondary tuned coaxial transmission line section 44, 48 to an output transmission line comprising inner and outer conductors 62 and 63 respectively. The output from the broad band tuned circuit is led downwardly and outwardly through the inner conductor 48 of the secondary tuned coaxial section by means of the transmission line 62, 63.

Referring particularly to Figs. 2, 3 and 4 for a disclosure of the present invention, there is shown at 12 a capacitor element hinged concentrically with the sections 10 and 11. In Fig. 4 the inner conductors 13 of the sections extend inwardly so that bearing blocks 14 are conductively pivoted thereon. The bearing blocks 14 are rigidly and conductively affixed to the plate 15. The plate carries two metal plates 16 and 16 (see Fig. 4) above it and another metal plate 17 below it, each being spaced from plate 15 by a sheet of mica or equivalent material. Each of the plates 16 and 16 are conductively attached to the filament connectors 18 by means of resilient straps 19. The purpose of the flexible straps 19 is to relieve mechanical strains on'the vacuum tube filament pins when the connectors are tightened. It will be readily appreciated, however, that the filament lead in system shown herein can also be applied to a heater-cathode type of tube, and the fact that this disclosure relates to a filament type of tube is in no way limiting in so far as the application of this invention is concerned. The plate 17 is attached to the connector by means of flexible strap 21. The purpose of the condensers comprising plates 15, 16, 16 and 17 is to place all three filament leads at the same radio frequency potential.

As shown in Fig. 4 the filament lead in conductors 22 are introduced through the inner conductors 13 and conductively connected to the plates 16 and 16.

Fingers 23 shown in Fig. 2 may be provided to connect the plate 15 to an adjustable neutralizing stub indicated at 22, the function of which is to provide a small amount of feedback from the plate to the filament of the vacuum tube shown. The function of stub 22, however, is more fully explained in the copending application above referred to and does not directly concern this invention.

Referring to Fig. 5, a source of radio frequency driving power is fed to the device by standard coaxial transmission line 24 and connected as shown to the section 10 and to stub 25 which is tunable by means of shorting disc 26. The inner conductors of the stub and the line 24 are tubular as shown. At a distance beyond the stub 25, suitable openings are provided in the inner and outer conductors of the extension of transmission line 24. A similar stub 27, tunable by means of shorting disc 28, is connected to the coaxial connector 11. Filament leads may thus be run through the hollow central conductors of stubs 25 and 26, sections 10 and 11, through the openings in the extensions of said sections, and thence to the filament connectors 18. The connector 20 is connected to the inner conductor as shown in Fig. 5 by means of a third condenser provided by plates '15 and 17 and the dielectric sheet therebetween. A source of filament heating power is shown in the filament transformer 29. The characteristic impedances of all coaxial lines shown in Fig. 5, including stubs 25 and 27, are equal.

The operation of the device depends upon the principle known as double stub matching which is illustrated in Fig. 6. According to that principle, an impedance Z can be transformed to a resistive value which is equal to the characteristic impedance Z0 of any transmission line provided that the equivalent shunt resistive component of impedance Z is not greater than /2 Z0 in value. No limitation need be placed on the equivalent shunt reactive component of impedance Z. If the spacing between the stubs is made approximately one eighth wave length or any odd multiple thereof, the match varies relatively slowly with frequency and a match is possible as long as the equivalent shunt resistive component of impedance Z is greater than /2 Z0. No restriction need be placed on Z if various lengths of line can be inserted between the right hand stub (Fig. 5) and the impedance Z.

In Figs. 5 and 6, the length between stubs has been made approximately /8 wave length at the operating frequency. The length between the transmission line and vacuum tube L has been chosen so that the input impedance of the vacuum tube, when operating as a grounded grid amplifier, can be transformed to a resistive value equal to the transmission line characteristic impedance by means of tunable stubs 25 and 27. This latter length is the distance between the point 30 in Fig. 5 and the vacuum tube terminals. It is indicated as L in Fig. 6. It has been found in practice, however, that while perhaps desirable to so design the length L as above described, such design is not critical within reasonable limits.

This invention is equally applicable to a triple stub method of impedance matching. It is also to be understood that the invention is not confined to the particular embodiment herein illustrated and described but embraces various modifications thereof such as are included in the scope of the following claims.

We claim:

1. Input structure for a grounded grid amplifier tube having cathode and filament terminals, comprising an input coaxial transmission line, matching stubs for said line and said tube, a section of transmission line interconnecting said matching stubs, said matching stubs and said section having hollow inner conductors with filament lead-ins therein, a connector comprising a first plate connecting said hollow inner conductors to the cathode terminal of said tube, an additional plate member connecting said lead-ins to a filament terminal, said plate members being spaced apart by dielectric means providing capacitive elements for cathode and filament bypass.

2. Input structure for a grounded grid amplifier tube having cathode and filament terminals comprising an input coaxial transmission line, matching stubs for said line and said tube, a section of transmission line interconnecting said matching stubs, said matching stubs and said section having hollow inner conductors with filament lead-ins therein, a connector hinged concentrically with said hollow inner conductors comprising a first plate connecting said hollow inner conductors to the cathode terminal of said tube, an additional plate member connecting said lead-ins to a filament terminal, said plate members being spaced apart by dielectric 6 means providing capacitive elements for cathode and filament by-pass.

3. Input structure for a grounded grid amplifier tube comprising an input coaxial transmission line, matching stubs for said line and said tube, a section of transmission line interconnecting said matching stubs, said matching stubs and said section having hollow inner conductors, a cathode and filament connector for said tube hinged to and concentric with the inner conductor of said section, and filament current supply leads located in the inner conductor of said section and at least one of said matching stubs.

References Cited in the file of this patent UNITED STATES PATENTS 2,218,309 Davies et a1. Oct. 15, 1940 2,272,060 Dow Feb. 3, 1942 2,426,185 Doherty Aug. 26, 1947 

